I don't know how IE7 stores the feeds internally, but if they're XML files, you can add that folder to WDS and it will happily crawl it. That said, the XML IFilter cannot guess what type of file it's reading, so it would just return the XML content.

Once we have the WDS SDK out, you will be able to use it to write an IFilter that finds the RSS store, extracts the data from it and populate the index with the correct properties.